1. Field of the Invention
The present invention relates to a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability for use in such applications as automobiles, home electric appliances and building materials.
2. Description of the Related Art
It has been the most common conventional practice to subject galvanized steel sheets for use in applications such as automobiles, home electric appliances and building materials to a phosphate treatment, a chromate treatment, and further, an organic coating treatment with a view to improving added values such as corrosion resistance and paintability of the galvanized steel sheets. However, the recent tendencies have been that the chromate-treated steel sheet, in particular, which may contain hexavalent chromium is avoided from environment problems, and there has been an increasing demand for phosphate treatment of the galvanized steel sheets.
With the conventional phosphate treatment of a galvanized steel sheet, however, sufficient corrosion resistance or paintability is not always achieved, and therefore various methods for the improvements have been proposed. For example, Japanese Patent Publication No. 60-34912 discloses a method in which after a phosphate film is formed, the film is treated with an inhibitor. Japanese Laid-Open Patent Applications Nos. 60-50175 and 8-13154 disclose methods of achieving coexistence of Ni, Mn and the like in a phosphate film.
The aforementioned methods, while giving certain effects, are not sufficient to meet with the recent severer requirements for corrosion resistance, and provides almost no improving effect of bare corrosion resistance in particular.
Japanese Laid-Open Patent Applications Nos. 1-312081 and 3-107469 disclose zinc phosphate films containing Mg. In these cases also, the corrosion resistance improving effect is not sufficient, with an insufficient paintability. Further, Japanese Laid-Open Patent Application No. 9-49086 discloses a method of forming a zinc phosphate film containing Ni and Mg, still suffering from the problem of insufficient corrosion resistance.
The present invention has an object to solve these problems and provide a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability.
For phosphate-treatment of a galvanized steel sheet, the present inventors have made various trials to cause Mg ion and Ni ion to be coexistent in large quantities in a treatment bath and to form a high-phosphate film having high contents of Mg and Ni which could not be achieved by the conventional art. As a result, it has been found that corrosion resistance and paintability can be improved by increasing the contents of both Mg and Ni in the phosphate film. It has been further found that a phosphate film having still higher Mg and Ni contents is achieved by coating an aqueous phosphate solution containing Mg and/or Ni, after the formation of the phosphate film containing Mg and Ni, and drying the resultant coated sheet without rinsing with water. These findings have led to a conclusion that, by maintaining the Mg and Ni contents in the resultant phosphate film within a specific range respectively, it is possible to obtain very good corrosion resistance and paintability so far unknown. It has been still further found that the same effect can be achieved by Mn in place of Ni.
The present invention has been completed on the basis of the aforementioned findings and the gist of the present invention is to provide a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability, comprising a phosphate film in an amount of at least 0.5 g/m2, containing at least 2 wt. % of Mg and at least 0.5 wt. % of Ni and/or Mn, with Mg and Ni and/or Mn being in total, formed on the surface of a steel sheet coated with zinc or a zinc alloy; and further provide a phosphate-treated galvanized steel sheet excellent in corrosion resistance and paintability, comprising a phosphate film in an amount of at least 0.3 g/m2, preferably at least 1 g/m2, containing at least 2 wt. % of Mg, at least 0.5 wt. % of Ni and/or Mn, with Mg and Ni and/or Mn being in an amount of at least 5 wt. % in total, formed on the surface of a steel sheet plated with zinc or a zinc alloy.
There is no particular limitation regarding the galvanized steel sheet used in the invention, and the invention is applicable to both pure zinc coated and zinc alloy coated steel sheet. Any zinc and zinc alloy coating method including electro-galvanizing, hot-dip galvanizing and vapor deposition may be applicable.
According to the present invention, the phosphate film formed on the galvanized steel sheet normally contains Zn dissolved from the zinc coating or coming from the phosphate treatment bath, but it is essential that the film contains Mg and Ni and/or Mn. The required ratios of these metal contents to the weight of the phosphate film as a whole are at least 2 wt. % for Mg, at least 0.5 wt. % for Ni and/or Mn, and at least 4 wt. % for the total of Mg and Ni and/or Mn. Any of these metal contents below the lower limits results in remarkable deterioration of corrosion resistance and paintability. Mg, Ni and/or Mn should preferably be at least 5 wt. % in total.
There is no particular limitation for the upper limit of the contents of the above metals. However, the content of Mg and Ni singly or in combination is limited up to about 10 wt. %, and Mg and Mn and/or Ni in total is limited up to about 15 wt. %. It is technically difficult to maintain their contents over these upper limits.
The phosphate film containing at least 4 wt. % of Mg and Ni and/or Mn in total must be present in a weight of at least 0.5 g/m2, below which no satisfactory corrosion resistance can be obtained. Also the phosphate film containing at least 5 wt. % of Mg and Ni and/or Mn in total must have a weight of at least 0.3 g/m2, below which no sufficient corrosion resistance can be obtained, and which should more preferably be at least 1 g/m2. Although there is no particular limitation on the upper limit, it should preferably be in general up to about 2.5 g/m2 when taking weldability into consideration.
Since Ni and Mn produce the same effects, the following description will cover only the case where Ni is used without using Mn.
The phosphate film containing Mg and Ni according to the invention can be obtained by the treatment using a phosphate bath containing Mg ion and Ni ion. Prior to such a treatment, it is desirable to perform a known pretreatment such as a titanium colloidal treatment or a brushing treatment. An example of the phosphate treatment bath may be illustrated by a bath prepared by adding Mg ion and Ni ion to a treatment bath containing Zn ion, phosphate ion, fluoride, an oxidizing agent (such as a nitrate, a nitrite or a chlorate). In this case, the concentration (weight percentage) of the metal ions relative to one Zn ion should be preferably about 10 to 50 for Mg ion, and about 1 to 10 for Ni ion. By using such a treatment bath, it is possible to form the phosphate film on the galvanized steel sheet by a spraying or dipping method, for example. However, by using such a method, if Mg and Ni are to coexist, their weight percentage in total in the resultant film is technically limited up to about 5% maximum. Trying to add them in a higher percentage is not only difficult, but also tends to cause defective precipitation of the film or production of much sludge.
In order to achieve coexistence of Mg and Ni in larger amounts, it is desirable to apply first a phosphate treatment using the bath containing Mg and Ni as described above, or a usual phosphate treatment using a bath not containing Mg or Ni, and then, to coat the thus treated sheet with an aqueous phosphate solution containing Mg and/or Ni, and drying the thus coated sheet without water rinsing to a sheet temperature of 90 to 150xc2x0 C. without water rinsing, thereby forming a composite phosphate film. As the aqueous solution to be coated, an aqueous solution of a primary phosphate of the metals (known also as dihydrogen phosphate salt or biphosphate salt) is preferable, and as the coating method, a roll-coating method is preferable. The coating may be applied to the both surfaces, or only to one surface of the sheet. Particularly for an automobile sheet, for example, it is also appropriate to coat only the surface of the sheet which, when used in an automobile, forms the inner surface required to have a high corrosion resistance.
When the above aqueous phosphate solution is coated, the weight of the phosphate film according to the present invention is the total weight of the primary phosphate treatment film plus the phosphate film formed by the aqueous phosphate solution. And the contents of Mg and Ni in the film are the total contents of Mg and Ni in both the primary phosphate treatment film and the subsequently coated phosphate film formed by the phosphate solution are expressed as percents of the total weight of both films and when the total contents of Mg and Ni and the total weight of both films are within the specified ranges according to the present invention, it is possible to obtain satisfactory corrosion resistance and paintability.
Mn may be used in place of Ni, as described above, and the same effects and advantages can be obtained also by simultaneously using Ni and Mn.